Long story short, I'm stuck with a Mac OSX 10.8 installation and do not have my iMac's original DVDs to install 10.6, which is required to install WC3 and SC1. Then I thought I could install another OS! I don't want to pay for W7/W8, so why not linux?

I've never touched linux.

Is this something that'd be relatively easy? (Is this more appropriate for the Apple forum?)

Do you have any friend who has a 10.6 installation? You could log in to your Battle.net, download the game client, install the game on that friend's computer, patch everything, and then copy everything over to 10.8.

Ubuntu is definitely a good choice for someone who has never used Linux before, but be warned that you may have more problems than if you installed Ubuntu on a standard (non-Apple) PC. Apple's firmware is notoriously buggy and often causes problems in this regard.

The best idea is probably to dual-boot OS X and Ubuntu until you get more comfortable with the system. You can use the standard Boot Camp utility built into OS X to install Ubuntu. From OS X's perspective, installing Ubuntu is exactly the same as installing Windows. There is an excellent install guide on Ubuntu's website that I highly recommend you read. It will walk you through the installation.

^ instead of winebottler (not updated anymore i believe) check out PlayOnMacit's built on Wine and if the game is popular (with the games you want to play are) it should be in the repository and will install an instance of the version of wine that has been tested and found to work best for that specific game. meaning a better over all performance for you

Can you use PlayOnMac or WineSkin as an alternative method / rout? I know I have a lot of Windows Games ported to my Mac using WineSkin; not sure about the linux side of things. Or how about a Virtual Machine like VirtualBox?